1. Field of the Invention
This invention relates to an accumulator type fuel injection apparatus and more particularly to a fuel injection control technique for activating an exhaust emission purifier in a diesel engine.
2. Description of the Related Art
Exhaust gases emitted from a diesel engine mounted in a bus, truck, etc., contain much particulate matter (PM) as well as HC, CO, NOx, etc. A diesel particulate filter (DPF) has been put into practical use as an after-treatment device of a diesel engine. The DPF captures PM, and burns and removes the captured PM with an external heat source and an oxidation catalyst for treating HC and CO. Recently, a continuous regeneration DPF has been designed wherein a catalyst that generates NO2 for supplying an oxidant to oxidize and remove PM is placed upstream of the DPF in place of the external heat source of the DPF, so as to continuously remove the PM on the DPF by the generated NO2. Further, insertion of an NOx catalyst has also been designed mainly for removing NOx in an exhaust passage.
It is known that such an oxidation catalyst, a continuous regeneration DPF, or an NOx catalyst can sufficiently function only in an activated state under an atmosphere at a relatively high temperature. Therefore, in a cool mode when an engine is just started, etc., it is required not only to quickly activate the oxidation catalyst, the continuous regeneration DPF, or the NOx catalyst, but also to always hold the oxidation catalyst, the continuous regeneration DPF, or the NOx catalyst in an active state.
Various techniques disclose providing the oxidation catalyst, the continuous regeneration DPF, or the NOx catalyst with a heat source such as an electric heater, so as to warm the oxidation catalyst, the continuous regeneration DPF, or the NOx catalyst at the starting time, thereby quickly activating the oxidation catalyst, the continuous regeneration DPF, or the NOx catalyst.
However, providing such a separate heat source leads not only to complication of structure, but also to an increase in costs and is not preferred.
On the other hand, in recent years, as a fuel injection control system of a diesel engine, a common-rail injection system has been put into practical use. The common-rail injection system injects a high-pressure fuel accumulated in an accumulator into a combustion chamber by electrically controlling opening and closing an injection nozzle. The diesel engine adopting the common-rail injection system has a feature that the opening timing of the fuel injection nozzle is variable and the fuel injection timing can be set as desired. This means that the common-rail injection system makes it possible to inject fuel not only in a compression stroke, but also in all strokes of suction, expansion, and exhaust.
In order to prevent an increase in engine operation noise and NOx caused by rapid explosive combustion at the initial stage of combustion, a technique for injecting a small amount of fuel at a low pressure at the initial stage of the fuel injection cycle (initial injection) has been developed and put into practical use in the field of the common-rail injection system.
Then, a technique has been developed using the feature of the common-rail injection system. In the technique, fuel for conducting main combustion is injected before injecting additional fuel in the expansion stroke and later (post injection). Then, the additional fuel is burnt by fire in the combustion chamber or the additional fuel is caused to react with a catalyst on an exhaust passage for raising exhaust temperature, thereby raising the temperature of an oxidation catalyst, a continuous regeneration DPF, or an NOx catalyst.
To conduct the post injection, penetration of the injected fuel is strong if high-pressure fuel is injected. Thus it is feared that the fuel might adhere to the cylinder liner wall, causing oil dilution, seizure, etc., to occur. Thus, a technique for injecting low-pressure fuel for minimizing the penetration of the injected fuel has been also designed for the post injection.
However, as described above, with respect to the common-rail injection system having two accumulators for accumulating high-pressure fuel and low-pressure fuel, respectively, the post injection with the low-pressure fuel should be conducted at a low pressure as much as possible. However, since the post injection temporally lowers the fuel pressure in a fuel passage communicating with a fuel nozzle or in the accumulator having the low-pressure fuel. Therefore, it is feared that it might be made impossible to maintain a sufficient fuel pressure, regardless that fuel is injected at a predetermined low pressure in the initial injection. The insufficient fuel pressure in the initial injection cannot accomplish a target combustion in the main combustion. This result is not preferred.
Thus, in the event that the post injection raises the exhaust temperature to quickly activate an oxidation catalyst, a continuous regeneration DPF, or an NOx catalyst, a problem arises as to how the fuel pressure at the post injection time is minimized as much as possible for preventing oil dilution, seizure, etc., while a sufficient fuel pressure is provided at the initial injection time to realize favorable main combustion.
It is therefore an object of the invention to provide an accumulator type fuel injection apparatus capable of providing a sufficient fuel pressure at an initial injection time of a main combustion and minimizing fuel pressure at a post injection time as much as possible to perform post injection for raising exhaust temperature.
According to a first aspect of the present invention, there is provided an accumulator type fuel injection apparatus comprising:
a first accumulator for accumulating high-pressure fuel having high pressure pressurized by a pump;
a fuel injection nozzle connected to the first accumulator via a fuel passage, the fuel injection nozzle for injecting fuel into a combustion chamber of an engine;
a change-over valve for communicating the high-pressure fuel in the first accumulator with the fuel passage and shutting off communication of the high-pressure fuel between the first accumulator and the fuel passage;
a second accumulator connected to the fuel passage downstream of the change-over valve via a branch passage, the second accumulator for accumulating low-pressure fuel having low pressure lower than the high pressure of the high-pressure fuel in the first accumulator;
a pressure control valve provided at one of the fuel passage downstream of the change-over valve and the second accumulator, the pressure control valve for adjusting fuel pressure in the fuel passage and the second accumulator;
an on-off valve adapted to control fuel injection from the fuel injection nozzle;
main injection control means for controlling the change-over valve and on-off valve to inject main fuel from the fuel injection nozzle during a predetermined period of time according to an operation condition of the engine; and
post injection control means for controlling the on-off valve to inject additional fuel from the fuel injection nozzle, after the injection of the main fuel by the main injection control means, thereby to raise exhaust temperature of the engine,
wherein the post injection control means injects the additional fuel so that the injection terminates at one of a first timing and second timing, whichever earlier,
at the first timing, the fuel pressure of the one of the fuel passage and second accumulator is lowered at a predetermined pressure lower than the high-pressure in the first accumulator, and
at the second timing, an exhaust stroke of the engine is completed.
In a common rail system having a first accumulator of high pressure and a second accumulator of low pressure, when main injection control means injects high-pressure fuel from the first accumulator after injecting low-pressure fuel from the second accumulator, the post injection control means injects additional fuel, thereby being burnt by flame in a combustion chamber or reacted with a catalyst in an exhaust passage to raise exhaust temperature. After termination of the fuel injection by the main injection control means, the main injection control means starts the additional fuel injection (post injection) so that the injection terminates at a first timing when the fuel pressure in either fuel passage or second accumulator lowers to a predetermined value lower than that of the high-pressure fuel or at a second timing when an exhaust stroke of the engine is completed, whichever earlier.
Accordingly, the post injection is started at the timing at which the fuel pressure in the fuel passage is higher than a predetermined low pressure, and controlled so that the fuel pressure is to be the predetermined low pressure at the timing at which the post injection ordinary ends. Thus, the predetermined low pressure is maintained when the main injection control means injects the low-pressure fuel (initial injection), and the initial pressure of the post injection becomes the minimum pressure for maintaining the predetermined pressure for the initial injection, so that penetration of the injected fuel is minimized as much as possible and the fuel is well prevented from adhering to the cylinder liner wall. Accordingly, while good main combustion is accomplished and oil dilution, seizure, etc., is well prevented, the exhaust temperature can be raised to quickly activate an after-treatment device.
Here, the reason why the post injection ends at the exhaust stroke end timing is that the post injection cannot contribute to exhaust temperature raising because the additional fuel cannot be exhausted toward an exhaust passage regardless of the post injection performed after an exhaust valve is opened. However, in this case, since the initial pressure of the post injection becomes the minimum pressure in case of the post injection performed before the exhaust stroke end timing, so that the penetration of the injected fuel is minimized as much as possible and the fuel is well prevented from adhering to the cylinder liner wall. In addition, since the fuel pressure of the fuel passage continues to be gradually reduced in an suction stroke after the exhaust stroke, the predetermined low pressure can be maintained at the initial injection timing.
According to a second aspect of the invention, there is provided an accumulator type fuel injection apparatus comprising:
a first accumulator for accumulating high-pressure fuel having high pressure pressurized by a pump;
a fuel injection nozzle connected to the first accumulator via a fuel passage, the fuel injection nozzle for injecting fuel into a combustion chamber of an engine;
a change-over valve for communicating the high-pressure fuel in the first accumulator with the fuel passage and shutting communication of the high-pressure fuel off between the first accumulator and the fuel passage;
a second accumulator connected to the fuel passage downstream of the change-over valve via a branch passage, the second accumulator accumulating low-pressure fuel having low pressure lower than the high-pressure fuel in the first accumulator;
a pressure control valve provided at one of the fuel passage downstream of the change-over valve and the second accumulator, the pressure control valve for adjusting fuel pressure in the fuel passage and the second accumulator;
an on-off valve for controlling fuel injection from the fuel injection nozzle;
main injection control means for controlling the change-over valve and on-off valve to inject main fuel from the fuel injection nozzle during a predetermined period of time according to an operation condition of the engine;
post injection control means for controlling the on-off valve to inject additional fuel from the fuel injection nozzle, after the injection of the main fuel by the main injection control means, thereby to raise exhaust temperature of the engine; and
pressure adjustment means for controlling the on-off valve to supply the high-pressure fuel in the first accumulator toward the fuel passage after the post injection control means injects the additional fuel by temporarily opening the on-off valve.
For example, in case much greater fuel is required at the post injection, even if the post injection lowers the fuel pressure lower than the predetermined low pressure, the high-pressure fuel in the first accumulator is temporarily supplied to the fuel passage so that the fuel pressure in the fuel passage can easily be restored to more than the predetermined low pressure.
Accordingly, at least the predetermined low pressure can be maintained at the time of the initial injection by the main injection control means. In addition, the post injection can be performed at the timing at which the fuel pressure in the fuel passage is lowered to the predetermined low pressure, so that so that penetration of the injected fuel is minimized as much as possible and the fuel is well prevented from adhering to the cylinder liner wall. Accordingly, while good main combustion is accomplished and oil dilution, seizure, etc., is well prevented, the exhaust temperature can be raised to quickly activate an after-treatment device.